Nuclear magnetic resonance (NMR) logging is a type of well logging that uses the NMR response of a formation to determine its porosity and permeability, providing a continuous record along the length of a borehole. NMR logging exploits the magnetic moment of hydrogen, which is abundant in rocks in the form of fluids. The NMR signal amplitude is proportional to the quantity of hydrogen nuclei present in a formation and can be calibrated to give a value for porosity that is free from lithology effects.
NMR logs provide information about the quantities of fluids present, the properties of these fluids, and the sizes of the pores containing these fluids. From this information, it is possible to estimate the volume (porosity) and distribution (permeability) of the rock pore space, the rock composition, the type and quantity of fluid hydrocarbons, as well as the hydrocarbon producibility.
Generally, NMR tools operate by imposing a static magnetic field on a geological formation. This magnetic field is traditionally referred to as the “static field” as it is usually independent of time and is given the symbol B0. A second magnetic field, which varies in time, is also applied. This field is typically designated as B1 and is traditionally called the “radio frequency field”. It is turned on and off at different increments, known as a pulse. This second, perturbing field is perpendicular to the static field, B0. The perturbing field moves the magnetization away from the thermal equilibrium. Generating the perturbing field takes a significant amount of energy, which may be in short supply downhole, especially if the logging tool is powered by batteries.
A particular type of energy-consuming RF pulse is a broadband saturation pulse that may commonly be used for downhole NMR logging of the spin-lattice relaxation time (T1) measurements. In a typical logging while drilling (LWD) NMR logging T1 logging sequence, the single broadband saturation chirp pulse uses about 25% more RF energy than an optimal saturation pulse may use. Furthermore, the chirp pulse uses a cumbersome calibration process that is time consuming and affects operation efficiency.